Transcript Fluid Flow - Physics 420 UBC Physics Demonstrations

Paul Drosinis
UBC Phys 420
Introduction
 Short history on fluid dynamics
 Why bother studying fluid flow?
 Difference between Newtonian and Non-Newtonian Fluids
 Laminar vs. Turbulent Flow and the Navier-Stokes
Equation
 Reynolds Number
Brief History
 Archimedes (285-212 B.C.)
– formulated law of
buoyancy and applied it to
floating and submerged
bodies
ischoolsfndiloy.wordpress.com
Brief History
 Isaac Newton (1642-1727) –
postulated laws of motion and
law of viscosity of linear fluids
 Frictionless fluids – many
problems solved by great
mathematicians (Euler,
Lagrange, Laplace, Bernoulli
etc.)
http://psychogeeks.com/isaac-newton/
Brief History
 Osborne Reynolds (1842-1912) –
classic pipe experiment illustrating
importance of so-called ‘Reynolds
Number’
http://en.academic.ru/dic.nsf/enwiki/191600
Why Study Fluid Flow?
 Widely applicable to many phenomena:
blood flow through arteries/veins,
automotive design, aeronautics
 Deeper understanding can be used to design
faster and more efficient ships/airplanes
Stress and Shear
 Stress: defined as
force per unit area
-has magnitude and
direction
 Can have both
normal and
tangential stresses
http://www.scribd.com/doc/10119418/Fluid-Mechanics-Lecture-Notes-I
Finding Newton’s Law of Viscosity
 We are going to model a ‘block of fluid’ as
many sheets stacked on top of one another
 In this way we can figure out how the shear
force is related to the viscosity
What is viscosity?
 Property of a fluid that
describes its ability to resist
flow
 It’s a measure of the internal
friction associated with this
flow
Substance
Viscosity(kg/m*s)
Air
0.02
Water
1.00
Milk
1.13
Blood
4
Olive Oil
90
Motor Oil
320
Stress and Shear
 Force in a fluid acts
along the surface of each
sheet and is proportional
to the relative velocity
http://www.britannica.com/EBchecked/topic/211272/fluid-mechanic
Finding Shear Force
y
x
Finding Shear Force
 Forces act parallel to the sheets
 If we talk about force per unit area, find that:
 As ℇ gets smaller, the difference becomes a gradient
Shear Stress
 If we model a body of fluid as composed of many thin
sheets, find that:
Velocity
Gradient
Stress
Viscosity
Finding Shear Force
 Constant of proportionality here is the viscosity: η
 What are its units?
Units of Viscosity
Newtonian vs. Non Newtonian
Fluid
 Linear dependence of shear stress with velocity
gradient: Newton’s Law of Viscosity
 Viscosity will change only if temperature or pressure
changes
 Don’t resist much when a force is applied
 Ex: water
Newtonian vs. Non Newtonian
Fluid
 Non-Newtonian fluids will change viscosity when a
force is applied
 Can cause them to become thicker or thinner
depending on the substance in question
DEMO!
Navier-Stokes Equation
 Set of non-linear partial differential equations that
describe fluid flow
 Also used to model weather patterns, ocean currents,
and airflow around objects
 Very difficult equation to solve
Navier-Stokes Equation
Rate Change in
Momentum Density
Viscous
Term
Pressure Gradient
Body
Forces
Reynolds Number
 Ratio of inertial forces to viscous forces in a fluid
 Describes the relative importance of each term
 Important factor in determining the transition from
laminar to turbulent flow
Reynolds Number
 Can be calculated from the Navier-Stokes equation
 More intuitively:
Reynolds Number
 η - viscosity
 ρ – density
 u - velocity
 d – characteristic length
Reynolds Number Units?
 Reynolds number is dimensionless!
Laminar Flow
 Fluid travels smoothly in similar paths
 No mixing between adjacent ‘sheets’ of fluid
 Sheets slide over one another
 All flow properties constant at any given point
(velocity, pressure, etc.)
Laminar Flow
https://wiki.brown.edu/confluence/display/PhysicsLabs/PH
Turbulent Flow
 Formation of eddies and vortices associated with high
Reynolds number fluids
 Flow becomes chaotic
 Complete description of turbulent flow still an
unsolved problem of physics
Turbulent Flow
http://www.colorado.edu/MCEN/flowvis/gallerie/2010/Team-1/FV_popup1-8.htm
Laminar vs. Turbulent Flow
 Fluids behave very differently depending on the value
of the Reynolds number
 Low Re – Laminar Flow
 High Re – Turbulent Flow
Questions?